The present invention relates to an injection controller for an internal combustion engine, and more specifically, relates to an injection controller for an internal combustion engine provided with a direct fuel injection valve, which injects fuel directly into the cylinder, and an intake passage fuel injection valve, which injects fuel into an intake passage.
Conventionally, certain internal combustion engines are provided with a direct injection valve (in-cylinder injection valve) for injecting fuel directly into the cylinder in addition to a port injection valve for injecting fuel into an intake port of an intake passage (for example, refer to Japanese Laid-Open Patent Publication No. 2002-364409). In such an internal combustion engine, the fuel injection is flexibly switchable in accordance with the engine operation conditions such that fuel injection is performed by only the port injection valve, fuel injection is performed by only the direct injection valve, and fuel injection is performed by both valves.
The direct injection valve includes a nozzle hole for injecting fuel. Since the direct injection valve is exposed to high-temperature combustion gas in the combustion chamber, deposits tend to adhere to the nozzle hole of the direct injection valve. When the direct injection valve is injecting fuel, the nozzle hole is cooled by vaporized fuel. However, when the direct injection valve is not injecting fuel, the nozzle hole is not cooled by vaporized fuel and the temperature of the nozzle hole increases such that deposits accumulate on the nozzle hole. Such deposits obstruct the injection of fuel from the nozzle hole of the direct injection valve. As a result, the shape of fuel mist may change (particle diameter increases) or the amount of injected fuel may decreases so as to be less than the amount required. There is concern in this case that misfire and unsatisfactory combustion may occur.
In the apparatus described in Japanese Laid-Open Patent Publication No. 2002-364409, some of the injected fuel of the port injection valve is allocated to the direct injection valve even during the period of operation in which the port injection valve is used. In this case, the nozzle hole is cooled by the vaporized fuel and the accumulation of deposits is prevented.
In the method for allocating some of the injected fuel of the port injection valve to the direct injection valve described above, when the required amount of injected fuel is relatively low, the portion of the injected fuel allocated to the direct injection valve is also relatively small. Therefore, an adequate cooling effect is not obtained, and the accumulation of deposits is not sufficiently prevented.
When a small amount of injected fuel is required and the fuel allocated to the direct injection valve falls below the appropriate minimum amount (minimum value ensuring linearity of the injected amount relative to valve open time of the injection valve), the direct injection valve cannot inject fuel normally. In this case, there is concern that misfire and unsatisfactory combustion may occur.